1. Field of the Invention
The present invention relates to an image reading device of the type having a one-dimensional image sensor such as a line sensor adapted to read an image on an image carrier such as an original document which is moved relatively to the line sensor in close contact therewith. The image reading device of the invention is suitable for use in, for example, facsimiles, copiers and image readers.
2. Related Background Art
An image reading device has been known in which successive line images on an original are focused through a contacting optical system on a one-dimensional line sensor having a length of several centimeters in the direction of the main scan, thereby enabling the line sensor to read the image carried by the original. This image reading device essentially requires a contacting optical system which has an optical path of a large length. In addition, the contacting optical system itself has a considerably large size. For these reasons, it has been difficult to obtain a compact design of the image reading device of the type mentioned above.
In another type of known image reading device, the line image is focused through a real-size optical system on an elongated line sensor having a length substantially equal to the length of the original as measured in the direct on of the main scan. It is rather easy for this type of image reading device to have a reduced size, because the size of the space accommodating the optical system is much smaller than that in the first-mentioned type of the image reading device. The real-size optical system can be constituted by, for example, converging optical fibers or a contact lens array.
The converging optical fibers and the contact lens array, however, are generally expensive so that the cost of the image reading device is raised undesirably.
Under these circumstances, the present applicant has proposed a contact-type image reading method in which an original carrying the image to be read is moved relative to a one-dimensional line sensor in direct contact therewith. This method is disclosed, for example, in Japanese Patent Application Laid-Open Publication Nos. 74262/1980, 45084/1981 and 122172/1981.
FIG. 1 is a schematic fragmentary sectional view of an essential portion of the contact-type image reading device. The device has a sensor 8 constituted by a one-dimensional line sensor which is arranged on a transparent substrate 11 so as a sheet of glass such as to extend in the direction perpendicular to the plane of the drawing.
The sensor 8 has a multi-layer structure constituted by a light-shielding layer 12 made of a light-shielding material such as a metal and placed on the transparent substrate 11, an insulating layer 13 made of an electrically insulating material and laid on the light-shielding layer 12, and a photoconductive semiconductor layer 14 formed on the insulating layer 13 and made of a photoconductive semiconductor material such as hydrogenerated amorphous silicon (referred to as "a- -Si : H", hereinunder) and CDs.Se. A pair of main electrodes 16 and 17 are formed on the semiconductor layer 14 through an intermediary of a semiconductor layer 15 which is made of a semiconductor material doped with a semiconductor impurity for the purpose of attaining an ohmic contact. The area between the pair of main electrodes 16 and 17 constitutes a light-receiving window 18.
In operation, a light L from a light source 30 enters through a window 19 formed in the transparent substrate 11 so as to illuminate an original P and is reflected by the image. The reflected light then impinges upon the window 18 of the sensor 8 thereby to produce a photoelectric current which flows between the main electrodes 16 and 17 through the semiconductor layer 14. This photoelectric current is detected as the reading signal.
According to this arrangement, the light from the light source 30 enters from the reverse side of the substrate 11. If the sensor 8 lacks the light-shielding layer 12, a portion of the light emitted from the light source will directly reach the sensor 8 in addition to the light reflected by the image. The light directly reaching the sensor 8 will produce a steady photo-electric current so as to generate a large amount of noise, resulting in an impractically low S/N ratio which is in this case the ratio between the signal photoelectric current produced by the reflected light and the noise produced mainly by the steady current. It is therefore essential that the sensor 8 has a light-shielding layer 12. In order to shield the light which would directly reach the sensor 8 without fail, the light-shielding layer 12 is required to have a sufficiently large light-shielding power. In addition, the light-shielding layer 12 preferably has a small thickness. To comply with these demands, the light-shielding layer is made of a metal or a similar material.
A reading resolution on the order of 4 to 8 lines/mm is obtainable when a clearance of about 0.1 mm is maintained between the original P and the sensor 8. In order to maintain the required resolution, the clearance between the original P and the sensor 8 has to be controlled precisely. Such a precise control of the clearance can be attained by, for example, forming a transparent protective layer 20 on the upper side of the sensor 8.
In general, the sensor of an image reading device is required to meet various demands such as reduced dark current, large S/N ratio and high linearity of the output current I.sub.p in relation to the incident light.
As explained before, the contact-type image reading device essentially requires a light-shielding layer 12. Hitherto, however, no specific consideration has been given to the potential of the light-shielding layer 12. In consequence, the light-shielding layer electrically floats and is held in an unstable state. This in turn makes the band state of the semiconductor layer 14 contacting the insulating layer 13 on the light-shielding layer 12 unstable, resulting in an unstable photoelectric current as the output.
The semiconductor layer 14, when formed from a-Si: .sup.H has n.sup.- conduction type so that the flat band voltage is slightly negative, as will be understood from a C-V curve which will be mentioned later. Therefore, a weak accumulation is caused at the boundary between the light-shielding layer 12 and the insulating layer 13. This inconveniently increases the level of the dark current resulting in a degradation of the quality of the reading output.
It is therefore strongly desired to eliminate the above-described problems encountered by the known image reading device.